A nightmare is a dream that causes strong emotional responses from the sleeper, typically fear/horror. The dream may contain situation(s) of danger, discomfort, or psychological or physical distress. Sufferers are usually woken in a state of distress, and might be unable to go back to sleep for a prolonged period.

Nightmares can have physical causes such as sleeping in an uncomfortable or awkward position, or having a fever; and psychological causes, such as stress or anxiety. Eating before bed, which triggers an increase in the body's metabolism and brain activity, is a potential stimulus for nightmares.

Occasional nightmares are commonplace, but recurrent nightmares can interfere with sleeping patterns and cause insomnia, and may require medical help. A recently proposed treatment consists of imagery rehearsal. This approach appears to reduce the effects of nightmares and other symptoms in acute stress disorder and post-traumatic stress disorder.

Medical investigation

Studies of dreams have found that about three quarters of dream content or emotions are negative.

One definition of "nightmare" is a dream which causes one to wake up in the middle of the sleep cycle and experience a negative emotion, such as fear. This type of event occurs on average once per month. They are not common in children under 5, but they are more common in young children (25% experiencing a nightmare at least once per week), most common in adolescents, and less common in adults (dropping in frequency about one-third from age 25 to 55).

Fearfulness in waking life is correlated with the incidence of nightmares.